1. Field of the Invention
The present invention generally relates to ink jet printhead apparatus and, more particularly, to an ink jet printhead having a multi-channel array actuation system suitable for producing spot size modulatable droplets.
2. Description of Related Art
In the parent of the present application, a method of forming tapered orifice arrays in fully assembled ink jet printheads was disclosed. In accordance with the teachings of that invention, an ink jet printhead body having a front side and a plurality of ink carrying channels axially extending therethrough is first provided. A cover plate to be mounted onto the front side of the ink jet printhead body is selected based upon the desired size of the orifices at the inner side of the cover plate and then mounted onto the front side of the ink jet printhead body to cover the ink carrying channels. For each orifice to be formed as part of the orifice array, first and second cover plate ablating light beams are generated and directed, at first and second angles, respectively, at specified locations on the front side of the cover plate which correspond to selected ones of the ink-carrying channels to form ink carrying channel communicating orifices extending through the cover plate, tapering outwardly therethrough and in respective communication with the selected ones of the ink carrying channels. In one particular aspect of that invention, the outwardly tapered orifice is formed to have a first opening at the front side of the cover plate and first and second openings in communication with the ink carrying channel at the back side of the cover plate.
FIG. 1 illustrates an enlarged, partial cross-sectional view of a cover plate 14' having a back side surface 14a' to be mounted to a main body portion of an ink jet printhead and a front side surface 14b'. Extending through the cover plate 14' is a tapered orifice 16 which may be formed using the techniques set forth in U.S. patent application Ser. No. 07/815,296 filed Dec. 31, 1991 and previously incorporated by reference. The tapered orifice 16 has a pair of inner apertures 18c and 18d and a single outer aperture 20c, all of which have approximately the same diameter.
It has now been discovered that the cover plate 14' is particularly useful in the construction of a spot size modulatable ink jet printhead having a multi-channel array actuation system. As is well known in the art, ink jet printing devices use the ejection of tiny droplets of ink to produce an image. To do so, a volumetric change in fluid contained in internal channels of the printhead is induced by the application of a voltage pulse to a piezoelectric material which is directly or indirectly coupled to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid and these are directed so as to force a small, fixed quantity of ink, in droplet form, outwardly through the discharge orifice at a fixed velocity. Alternately, a voltage may be applied to a resistive element disposed in the fluid, thereby heating the fluid contained in the internal channels until expansion of the heated ink causes the ejection of a droplet of ink therefrom. However ejected from the ink jet printhead, the droplet strikes the paper at a specified location related to the image being produced and forms an ink "spot" having a diameter directly related to the volume of the ejected droplet.
As should be readily appreciated by one skilled in the art, the modulation of spots formed by an ink jet printhead may be used to produce gray scale images. In the past, attempts to produce a gray scale image using an ink jet printhead were directed to the modulation of spot density. To do so, the ink jet printhead creates various shades of gray by varying the density of the fixed size ink spots. Darker shades are created by increasing spot density and lighter shades are created by reducing spot density. Producing a gray scale image in this manner, however, reduces the spacial resolution of the printer, thereby limiting its ability to produce finely detailed images. Furthermore, the more levels added to the gray scale, the greater the resultant degradation of the printer's spacial resolution. A second proposed solution has been to direct multiple droplets at a single location on the sheet of paper to form variably sized spots. While such a method can produce the variably sized spots necessary to produce a gray scale image, such a technique tends to reduce the operating speed of the printer to an unacceptably low level. Furthermore, this technique also tended to produce elongated or elliptical dot patterns.
It can be readily seen from the foregoing that it would be desirable to provide an improved drop-on-demand type ink jet printhead configured such that the size of ink spots produced thereby is readily modulatable to produce a gray scale. It is, therefore, an object of the present invention to provide such an improved drop-on-demand type ink jet printhead utilizing a tapered orifice array in which each orifice has a single opening on a front side surface thereof and a pair of openings on a back side surface thereof in combination with a multi-channel array actuation system.